The present invention relates to mobile terminals adapted for reproducing sound or music files, respectively, particularly reproducing MIDI (Musical Instrument Digital Interface) data files.
MIDI is a data format, which does not contain sampled audio data like for instance “.wav”-files, but a specification on how the sound is to be rendered. A MIDI file can be regarded as a sheet of music in an electronic legible format. It contains information about the soundtrack and the devices being used and the acoustical parameters which have to be considered when reproducing the score represented by the data stored in the respective MIDI file. The collective term acoustical parameter denotes statements defining for instance the pitch, the note or rest values, respectively, the loudness level, the tempo, the timbre or special effects like vibrato or reverberation.
To turn a MIDI file into sound, the information present in the MIDI file has to be interpreted and formed to data representing a sampled, digital sound. Therefore, a so-called “MIDI synthesiser” is used, which renders the score of the MIDI file to sample data. like e.g. those used in a mono or stereo “.wav”-file. “The MIDI synthesiser may be implemented in software in a digital signal processor or in a separate dedicated hardware. The rendering of the score is usually based on so-called wave tables, which contain sound samples of an instrument, like e. g. of a piano, in form of digitally sampled data. On mobile terminals, like for instance mobile phones, PDAs (Personal Digital Assistants) or the like, pieces of music are preferably kept in store in form of MIDI files, as the size of MIDI files is extremely small compared to files containing sampled audio data. A PCM (Pulse Code Modulation) format audio file, like for example a “.wav”-file uses up to 10 Megabyte per minute of music while the same music can be stored in a MIDI file of less than 10 Kilobyte. This is possible like already mentioned above, as the MIDI file contains only the instructions needed by a MIDI synthesiser to reconstruct the respective sound and not the sound data itself.
On mobile terminals, MIDI files can be replayed for entertainment or be used as ringer or alarm signals to indicate incoming calls, received messages or other events.
The MIDI files can be sent and received by a mobile terminal in form of a SMS (Short Message Service) or MMS (Multimedia Messaging Service) type of message.
An electroacoustic reproduction circuitry of a mobile terminal allows the reproduction of the score stored in a MIDI file on the basis of the mono or stereo sampled data rendered from the score by the MIDI synthesiser. Depending on the characteristics of the electroacoustic reproduction circuitry, particularly the loudspeaker used, there are certain restrictions, like e.g. the frequency response, the dynamic range, and the maximum allowable amplitude of the sound signal which are to be considered when reproducing a score from a MIDI file. Although mainly the thresholds given by the respective restrictions are not to be exceeded without degrading the quality of the reproduced sound signal, many specific applications like ringer or alarm signals, require to reproduce a score close to the thresholds for obtaining the maximum possible output level.
As the sound signal rendered from a MIDI file depends on the algorithms and wavetables of the MIDI synthesiser used, critical values of the sampled data obtained when rendering the score, like for instance the maximum amplitude or maximum dynamic range, cannot be predicted from the data stored in a MIDI file directly. The <BR><BR><BR> volume and/or the dynamic range of as audio signal reproduced from a MIDI file are commonly adjusted by a dynamic compressor or limiter. These are signal processors implemented in software or hardware modifying the audio signal in the course of repreduction based on the current and past values, ignoring critical values <BR><BR><BR> like e. g. a peak @ amplitude occurring in the future of the playback. The consequence is a degraded fidelity of the sound reproduction due to the audible or annoying artefacts implemented.